Sheet material for electrical insulation

Abstract: An electrically insulated coil, characterized in that, an electrical insulation layer (2) wound around an electrical conductor (1) for the electrically insulated coil is formed by laminating layers of glass flakes having thickness of 0.2∼ 1 µm and diameter of 40∼ 250 µm on a backing member of a sheet like electrical insulation material and by impregnating with a semi cured thermosetting resin. Thereby an electrically insulated coil having an excellent partial discharge resistance as well as having a high dielectric break down strength even when subjected to a large mechanical distortion is provided.

Description: Background of the Invention The present invention relates to a sheet material for electrical insulation suitable for an electrically insulated coil to be used for a high voltage electrical machine and requiring in particular a high mechanical strength. Electrically insulated coils for conventional electrical machines, for example, dynamo electric machines are produced either with a dry electrical insulation tape constituted by a glass cloth and a laminated mica or flake mica by winding the same around a coil conductor, impregnating a thermosetting resin thereinto and thereafter by forming, heating and curing, or with a semi-cured prepreg tape formed by impregnating in advance a thermosetting resin into the electrical insulation tape by winding the same around a coil conductor, forming the same and thereafter heating and fully curing the same. In the above explained conventional electrically insulated coils, their dielectric strengths are designed to be primarily shared by the laminated mica and their mechanical strenghts are designed to be primarily shared by the flake mica or the glass cloth. However, the flake mica has a drawback that both the dielectric resistance and mechanical strength are comparatively low, as well as the glass cloth has a drawback that the dielectric resistance is low because of a lack of a barrier effect against an electric discharge streamer, therefore there was a problem that even the composite structure thereof with the laminated mica having a low mechanical strength cannot provide a sufficient dielectric resistance. As a measure for improving the above indicated mechanical strength, a glass flake sheet is known as, for example, disclosed in JP(U)-B-48-4851(1973) or in GB-A-1 342 425, respectively. The glass flake sheet is formed like a paper made by using scale like pieces prepared by pulverizing a thin glass and a backing member such as a glass cloth so that the thus formed glass flake sheet shows a better mechanical strength than the laminated mica sheet formed in the like manner. However, the conventional glass flakes are made of glass film prepared by an inflation process wherein a molten glass is inflated into a large ball and cooled, thereafter pulverized into flakes of glass film, therefore the size of the flakes is large, the thickness thereof is more than 5 µm and the diameter thereof is about 400 µm, further, the mutual adhesion of the flakes when laminated is insufficient possibly because of the spherical surface configuration of the respective flakes. Therefore a sufficient barrier effect of the flakes as indicated above cannot be obtained which developed drawbacks of poor partial discharge resistance of the electrical insulation sheet. Prior art document DE-C-972 678 discloses the combined use of a glass cloth tape and an inorganic material in bulky shape such as flake made of, for example, glass having a size which is smaller than the clearance diameter of the glass cloth tape so as to permit filling thereof into the clearances. DE-C-972 678 does not disclose the use of the glass flake having the specific size as extremely thin and flat glass flakes of the order of 0.2 - 1 µm thick. DE-B-1 005 613 discloses a production method of a U shaped insulator casing which is formed by laminating glass inforced clothes with thermosetting silicon resin. The citation nowhere discloses the use of the glass flakes of the specific size. JP-A-2-59452 shows a production method of a porous glass used like an ordinary plate glass by making use of a sol-gel technique. The citation nowhere discloses extremely thin and flat glass flakes. CH-B-607 412 describes a prepreg for a high voltage coil which is formed by impregnating a fabric mat such as glass fabric cloth with epoxy acid anhydride resin. The citation nowhere discloses the use of very thin and flat glass flakes. Summary of the Invention The object of the present invention is to provide a sheet material for electrical insulation using glass flakes having a higher dielectric resistance and partial discharge resistance than those of the conventional ones. This object is accomplished with a sheet material according to claim 1. Dependent claims are directed on features of preferred embodiments of the invention. A feature of the present invention is the use of glass flakes having the thickness of 0.2 - 1 µm and a diameter of 40 - 250 µm which show a high mechanical strength and an excellent dielectric resistance in comparison with the conventional ones. These glass flakes are, for example, prepared by pulverizing glass films produced by sol-gel processing. By making use of the electrical insulation sheet material using the glass flakes according to the present invention, the manufacture of electrically insulated coils having a high voltage withstanding property as well as an excellent mechanical strength is realized. The sol-gel processings referred to above are those wherein a metal alkoxide represented by the following chemical formula (1) is used as a raw material ; M(OR)n wherein M stands for a metal atom having n-valent and R a low alkyl group, with addition of water to the raw material colloidal solution is produced through hydorolysis and the subsequent condensation reaction, and thereafter a sol-gel tranfer is caused. The thus obtained raw material in a gel state and wet condition is formed on a flat plate and dried, and thereafter it is sintered by gradually heating at a predetermined temperature, for example, in case of silicon alkoxide at a temperature of 500 ∼ 1000°C. Si, Al, Ti and Ta can be used for the metal M in the above chemical formula, however Si is preferable for the metal M. When silicon alkoxide is used which is determined by selecting Si for the metal M, SiO 2 glass flakes having high purity can be obtained in comparison with conventional ones. The sol-gel processings as referred to above include the following advantageous features: a glass having a pure composition can be obtained, a large freedom is permitted for determining the shape of the glass film formed and the glass having film thickness of about submicrons can be freely manufactured. In particular, with the sol-gel processings glass flakes having a thin film thickness, a small diameter and a flat shape can be manufactured which could not be manufactured with the conventional inflation processings. An example of such glass flakes manufactured by the sol-gel processings is SG glass flakes (product of Nihon Plate Glass Manufacturing Company). The electrical insulation sheet material is manufactured by dispersing the above glass flakes in the same manner as a paper making on a backing member of glass cloth. Further, when the thus manufactured electrical insulation sheet is impregnated by a thermosetting resin diluted by such as a solvent, then the solvent is dried and thereafter heated at a predetermined temperature to render the impregnated resin into a semi-cured state (to render it into B-stage), a glass flake prepreg is obtained. The type of the resin to be impregnated is not limited if such can be used for manufacturing such as the ordinary glass prepregs, however, resin of epoxy series are preferable in view of their dielectric properties and heat resistances. Further, any solvents can be used if such can dissolve sufficient amount of resin to be impregnated. A glass cloth is most common as the backing member for the above glass flake prepreg, however, woven or non woven fabrics of such as liquid crystal polymer, for example alamide, and polyester can be used depending on application purposes. Further, since the glass flakes exhibit a high tensile rupture distortion in comparison with mica flakes, therefore when these glass flakes are laminated like the laminated mica, a sheet like electrical insulation material having a high mechanical strength can be obtained. For manufacturing such electrical insulation sheet, glass flakes having the thickness of 0.2 ∼ 1 µm and a diameter (the size of the flake) of 40 ∼ 250 µm are preferable. The glass flakes exhibit a high dielectric resistance and a good partial discharge resistance. It is not necessarily clear why the electrical insulation sheet and prepreg using the glass flakes according to the present invention show superior properties with regard to the above resistances, however, since the thickness of the glass flakes is in the order of submicrons, the diameter (the size of the flake) is as small as below 250 µm and the layer formed by laminating the glass flakes is dense, it is understood that the barrier effect of the glass flakes against electric discharge streamer in vertical direction is increased. In particular, the flatness of the glass flakes prepared by the sol-gel processings is desirable in comparison with ones prepared by the conventional inflation processings, thereby the adhesion between the glass flakes when laminated is superior and occurrence of microscopic voids is limited, therefore it is understood that the above excellent properties are realized. Brief Description of the Drawings Fig.1 is a graph illustrating V-t characteristics of electrical insulation sheets including one according to the present invention ; Fig.2 is a schematic cross sectional view of an electrically insulated coil used in an embodiment of the present invention. Fig.3 is a schematic cross sectional view of another electrically insulated coil having two electrical insulation layers used in another embodiment of the present invention. Fig.4 is a view for comparing bending distortion - dielectric break down characteristics of electrically insulated coils including one according to the present invention ; Fig.5 is a graph illustrating bending distortion characteristics of electrically insulated model coils using the glass flakes according to the present invention and the laminated mica ; Fig.6 is a schematic diagram of a rotor and a stator of a turbine generator ; and Fig.7 is a schematic diagram of a tokamak type nuclear fusion device. Detailed Description of the Embodiments [ Embodiment 1 ] Si series glass flakes (SG glass flakes, product of Nihon Glass Plate Manufacturing Company, having the thickness of 0.5 µm and the diameter of 40 ∼ 150 µm) prepared according to the sol-gel processings are dispersed in a solution (solution of methyl ethyl ketone / ethyl cellosolve) containing 2wt% of phenoxy resin which functions as a binder, a film of the glass flakes is formed on a backing member of glass cloth (thickness of 35 µm) in the same manner as paper making, after removing the liquid the resultant sheet is dried for 5 ∼ 10 minutes at 120°C and dipped into and impregnated with a solution of methyl ethyl ketone / triol containing 20wt% resin mixture of novolak type epoxy resin and bisphenol epoxy resin (1/1 mixture) blended with 3wt% BF 3 complex functioning as a curing catalyst and heated for 10 minutes at 95°C for rendering the resin into a semi cured condition, thereby a glass flake prepreg having thickness of 200 µm is obtained. It is observed that the resultant prepreg contains 53wt% resin. Two pieces of the glass flake prepregs are stacked, pressed, formed and cured for 1 hour at 160°C, thereby sheet (A) having the thickness of 260 µm is obtained. For the purpose of comparison, sheet (B) having a thickness of 320 µm is prepared which is produced in the same manner as above from two pieces of prepregs (thickness of 260 µm) by making use of glass flakes (thickness of 5 µm and average diameter of 400 µm) prepared according to the conventional inflation processings. Further, sheet (C) having a thickness of 200 µm is prepared which is produced in the same manner as above from one piece of a laminated mica prepreg of the thickness of 280 µm. Fig.1 shows the measurement results of V-t characteristic on the above respective sheets. As seen from Fig.1 the glass flake sheet (A) according to the present invention exhibits an extremely superior V-t characteristic in comparison with the conventional glass flake sheet (B) and further exhibits a comparable characteristic with that of the laminated mica sheet (C). [ Embodiment 2 ] In the present embodiment, electrically insulated coils as illustrated in a schematic cross sectional view in Fig.2 are produced by making use of prepreg tapes prepared in the same manner as in the embodiment 1. The respective prepreg tapes are wound around respective coil conductors 1 by a predetermined number to constitute electrical insulation layers 2 respectively, then the respective assemblies are placed in a metal mold, formed by pressing and heating and cured to produce respective electrically insulated coils. Fig. 4 shows the relationship between bending distortion and dielectric break down voltage of the thus produced respective electrically insulated coils. With regard to the electrically insulated coil using the laminated mica tape (C) when the bending distortion exceeds 0.3%, the dielectric break down voltage suddenly decreases, on the other hand, with regard to the electrically insulated coil using the glass flake tape (A) according to the present invention no substantial decrease of the dielectric break down voltage was observed even when the bending distortion reaches upto 0.5%. Further, it was observed that the electrically insulated coil formed by the glass flake tape (A) according to the present invention also shows a superior characteristic to that formed by the flake mica tape (D). Further, in Fig.5 bending distortion characteristics of the electrically insulated model coil using the class flake tape (A) and the electrically insulated model coil using the laminated mica tape (C) are shown. As seen from the drawings it will be understood that the glass flake tape (A) exhibits a superior distortion characteristic compared to that of the laminated mica tape (C). Further, characteristics of electrically insulated coils having two layered electrically insulated layer as illustrated in Fig.3, namely using a combination (E) of laminated mica + flake mica and using another combination (F) of laminated mica + the glass flake according to the present invention are also indicated in Fig.4. Further, in the electrically insulated coil as shown in Fig.3, the laminated mica prepreg tape is used for the inner electrical insulation layer 3 and the respective prepreg tapes are used for the outer electrical insulation layer 4. In this instance too, it is determined that the electrically insulated coil formed by the glass flake prepreg tape according to the present invention exhibits an excellent characteristic and the reduction of the dielectric break down voltage is extremely limited upto a bending distortion of 0.5%. According to the present embodiment, by forming the electrical insulation layer of the laminated mica having an excellent partial discharge resistance at the coil conductor side where electrical field is concentrated and by forming thereover the electrical insulation layer of the glass flake according to the present invention having an excellent mechanical characteristic, an electrically insulated coil having an excellent partial discharge resistance as well as excellent mechanical characteristics are obtained. According to the above investigation results, it is understood that with the glass flake tape according to the present invention a partial discharge resistance like the laminated mica can be realized as well as a high dielectric break down voltage is maintained even at bending distortion of 0.5%. In the above embodiments, the electrically insulated coils are produced by making use of the respective prepreg tapes, however it is not necessary to use such prepreg tapes. For example, the electrically insulated coils can be produced by making use of dry tapes which have not been impregnated by a resin, in that after winding such dry tapes around the coil conductor the dry tapes can be impregnated by a proper resin. In particular, by impregnating the resin in vacuum voidless electrically insulated coils having a higher reliability are obtained. Further, in the above embodiments, the glass flakes prepared by the sol-gel processings are used, however glass flakes prepared by the conventional method can be used if the thickness and the diameter thereof are controlled within the ranges according to the present invention. Nevertheless the glass flakes prepared by the sol-gel processings are still preferable. Further, when glass flakes of which surfaces are treated by a coupling agent such as amino silane or an impregnation resin into which a coupling agant is mixed is used, the mechanical strength of the resultant sheet and the prepreg can be further increased. Fig.6 is a schematic diagram of a rotor and a stator of a turbine generator, wherein field windings 7 are disposed in a rotor core 10 of a rotor 5 and the ends of the field windings 7 are clamped by a retaining ring 8. Further, stator windings 9 are disposed in a stator core 6 located outside the rotor 5. The field windings 7 are subjected to stresses due to centrifugal force. When the generator frequently repeats starting and stopping, the field windings 7 as well as the stator windings 9 are frequently subjected to such stresses. When the glass flake electrical insulation tapes according to the present invention are applied to the electrical insulation layers for the field windings 7 and the stator windings 9, electrically insulated coils having an excellent dielectric resistance as well as a high mechanical strength are obtained, thereby a highly reliable dynamo electric machine can be provided. Fig.7 is a schematic diagram of a tokamak type nuclear fusion device wherein along the circumference of a vacuum vessel 11 for confining plasma supported by an upper supporting stand 14 and a lower supporting stand 15, poloidal magnetic field coils 13 and toroidal magnetic field coils 12 are disposed so as to perpendicularly cross each other for generating a magnetic field which holds the plasma at the center of the cylindrical vacuum vessel 11. The magnetic field is generated by flowing a large current through the respective coils 12 and 13, accordingly large stresses act on the respective coils. When the glass flake electrical insulation tapes according to the present invention are applied to the electrical insulation layers for the poloidal magnetic field coils 13 and the toroidal magnetic field coils 12 which can withstand such stresses, electrically insulated coils having an excellent dielectric resistance as well as a high mechanical strength are obtained, thereby a highly reliable nuclear fusion device can be provided. According to the present invention, an electrically insulated coil having an excellent partial discharge resistance as well as having a high dielectric break down strength even when subjected to a large mechanical distortion is provided which is suitable for an electrically insulated coil for an electrical machine which is used in a severe operating condition subjected to a large stress under a higher electrical field intensity.

Claims: A sheet material for electrical insulation, characterized in that said sheet material is formed by laminated layers of glass flakes having a thickness of 0.2 - 1 µm and a diameter of 40 - 250 µm on a backing member of a sheet like electrical insulation material. A sheet material for electrical insulation according to claim 1, wherein said glass flakes are made by sol-gel processings. A sheet material for electrical insulation according to claim 1 or claim 2, wherein said sheet of electrical insulation material is made of woven and non-woven glass fabrics. A sheet material for electrical insulation according to one of the preceding claims, wherein said sheet material is further impregnated with a semi-cured thermosetting resin to form a prepreg sheet material. A sheet material for electrical insulation according to claim 4, wherein said prepreg sheet material is used to be wound around an electrical conductor to form an electrically insulated coil. Sheet material for electrical insulation according to claim 4, wherein said prepreg sheet material is used to be wound around an electrical conductor to form at least one of a stator coil and a rotor coil for a dynamo electric machine. Sheet material for electrical insulation according to claim 4, wherein said prepreg sheet material is used to be wound around an electrical conductor to form at least one of a toroidal coil and a poloidal coil for a nuclear fusion device.

Inventors: HIGASHIMURA YUTAKA, KAMIYA HIROYUKI, MIYAO HIROSHI, ONODA MITSURU, SATOO TAKANORI, TAKAMURA MAKOTO, HIGASHIMURA, YUTAKA, KAMIYA, HIROYUKI, MIYAO, HIROSHI, ONODA, MITSURU, SATOO, TAKANORI, TAKAMURA, MAKOTO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1994
Main classification: H02K15
IPC codes: H01B17, H02K3, H01F27, H01B3, H05H1